Abstract

A novel reconfigurable architecture based on slow-wave propagation in integrated optical ring resonators is proposed for the realization of variable optical delay lines. A continuously variable delay is achieved by combining a coarse discrete (digital) delay, provided by a coupled resonator slow-wave structure, with a fine continuous (analog) delay given by a cascaded ring-resonator phase-shifter. The reflective configuration of the structure enables a simple, accurate and robust tuning of the delay and provides a footprint reduction by a factor 2 with respect to conventional coupled resonator optical waveguides. Proof-of-concept devices realized in 4.4% silicon oxynitride waveguides and activated by a thermal control are discussed. Experimental results demonstrate, in both spectral and time domain, a continuously variable delay, from zero to 800 ps (2 bit fractional delay), on a 2.5 Gbit/s NRZ signal, with less than 8 dB insertion loss and less than 5 mm2 device footprint.

Photograph of (a) the SiON waveguide cross section and (b) the first rings of a SWDL realized with directly coupled ring resonators. The diameter of the rings is approximately 1.24 mm, corresponding to FSR=50 GHz.

(a) Theoretical group delay of a ring resonator phase-shifter with FSR=50 GHz and r2=0.65; (b) Experimental eye pattern of a 2.5 Gbit/s NRZ optical signal delayed by the resonator shown in (a) for three different detuning of the ring’s resonant frequency with respect to the signal carrier: 3.5 GHz (b1), 2 GHz (b2) and 0 (b3).